The invention relates to a mold for injection molding and diecasting machines with two closable mold parts, which form between them a die cavity, probably using mold mandrels and/or mold slides, and of which one mold half accommodates at least one ejector, with the ejector, the mandrel and the slide being seated slidable as movable inserts in straight-line guides of the mold parts.
To be able to manufacture and subsequently eject subjects with hollows or undercuts by means of injection and diecast molding, mandrels, slides and ejectors are required, which must be inserted relatively displaceable in the mold parts. To this, there have as yet been straight-line guides with slide faces parallel to the guide direction in the mold parts and guide surfaces of the inserts also parallel to the guide direction, with straight-line guides and guide surfaces usually manufactured with a clearance fit (H7/g6). Due to the clearance-oriented manufacture there has in fact been a guide clearance between the respective maximum and minimum clearance, permitting a slidability of the parts, and intended to ensure the development of a sufficient lubrication film during mold operation and to prevent the penetration of the hot liquid melting during injection molding. In practice, these parallel slide systems are subjected to considerable wear, thus endangering the functional safety of the guides, leading to a deterioration in quality of the subjects due to the development of fins, requiring regular maintenance and lubrication cycles and resulting in a frequent exchange of the precise and expensive guide parts or the exchange of the whole mold after a relatively short service life. Particularly in case of non-lubricated operation of the guide systems, as it is frequently demanded in medical technology, in the electronic industries or in the automotive industries, there is an increase in wear, which leads to production losses and expensive repairs in the area of the mold.
It is therefore the objective of the invention to create a mold of the above mentioned type characterised by particularly long wearing and yet trouble-free guiding devices, which is also extremely suitable for operation without lubricants.
The objective of the invention is achieved in that the straight-line guides leading into the die cavity are provided with a taper narrowing in closing direction, preferably with a taper angle of 1xc2x0 at maximum, and in that the guide surfaces of the inserts assigned to the straight-line guides run at a slant relative to the guide direction that is matched with this taper. Due to this slight taper the straight-line guide proper is not adversely affected, but the result are, on the one hand, extremely small guide gaps in closed position, which are, on the other hand, quickly widening as soon as the closed position is left. Therefore, opening and the closing are effected at a comparably large guide gap, usually up to 0.3 mm in width, leading to a low-contact and low-strain relative movement between the guide and the guide surface, thereby largely eliminating wear. Thus, of course, wear in the guide area of the inserts can be minimised, too, and a controlled sealing against the die cavity over the whole servcie life of the mold is ensured. Moreover, due to the increased gap width during closing and opening, the omission of lubricants becomes less problematic and the requirements to surface roughness and surface hardness become less stringent, which helps to simplify the very expensive working sequence when manufacturing the parts of the guide system.
It is particulary favorable, when, in closed position, there is a residual gap, predetermined by a mechanical stop, between the straight-line guides and the guide surfaces, preferably in the width range of 0.005 to 0.020 mm. By means of this residual gap it can be avoided that the inserts in the straight-line guides get jammed due to expansion or wedged due the closing forces, without the residual gap opening evident leaks to the die cavity, thus affecting the quality of the subjects due to the formation of fins. In spite of these minimum residual gaps, however, the inserts remain smooth-running and extremely long wearing due to the taper of the guides and the guide gap widening immediately together with the opening movement.
To be able to take up the very high closing forces without the danger of damage even in case of small mandrels, an additional straight-line guide separated from the die cavity is assigned as a secondary guide to the straight-line guide leading into the die cavity as the primary guide, with this latter having a greater taper angle compared with the seconary guide. This bigger and more rigid secondary guide is able to endure the accordingly high strains of the closing unit of the injection molding or diecasting machine and protects the more delicate primary guide from excessive strain. The taper angle of the pimary guide is at least 0.01xc2x0 greater than the taper angle of the secondary guide, to ensure that in none of the possible sliding positions damaging or wearing forces are transmitted to the mandrel projecting into the die cavity, as in closed position the residual gap of the primary guide is at best equal to the residual gap of the secondary guide, however, during opening or sliding, the gap of the primary guide is always wider than the guide gap of the seondary guide.
To facilitate the fabrication of the mold, particularly with respect to preserving the residual gap, the straight-line guides of the mold parts and the guide surfaces of the inserts are first finished with respect to their taper and the inserts are inserted into the straight-line guides without any gap, whereupon the guide surfaces of the inserts, prefabricated with a sufficient overlength are cut to length according to the longitudinal displacement, which has been established by calculation, for clearing the residual gap, and joined at the mechanical stops for securing the longitudinal position determining the residual gaps. Thus, fabrication and preservation of the residual gap is not effected in a complicated way via the diameter of the guides and/or guide surfaces, but via an axial displacement of the inserts relative to the straight-line guides according to an exactly calculable axial distance, which leads to an accurate and inexpensive determination and fixing of the residual gap. Due to this calculated axial displacement for determining the residual gap, relatively high length tolerances at small taper angles remain of little significance, as they cause only minimum gap changes, with gap determination and gap fabrication as accurate as possible being preconditions for a perfect and troublefree function of the guides.